The present invention relates to optical beam-splitters, and more particularly to an improved x-prism beamsplitter.
Beamsplitters are optical elements used in a wide variety of optical devices and systems. One type of beamsplitter is the x-prism. One exemplary type of optical system employing an x-prism is the helmet visor display.
A problem with conventional visor display apparatus is the lack of brightness which might limit the usefulness of the display in daytime or near-dusk conditions. A conventional helmet visor display is illustrated in FIG. 1, and employs a cathode ray tube 20 (CRT) which generates an image which is passed through a lens 22 and split by an x-prism 24 into an image for both eyes. Fold mirrors 34A, 34B and optical combiners 36A, 36B are employed to redirect the image light from the x-prism 24 to the wearer's left and right eyes. The function of the fold mirrors 34A, 34B is simply to redirect the light from the x-prism toward the combiners 36A, 36B. The function of each combiner 36A, 36B is two-fold: (1) to collimate the image made from the CRT 20 and lens 22 so that the virtual image appears at optical infinity; and (2) to act as a beam-splitter so that the CRT image may be overlaid with the outside scene. The combiner is actually a mirror/visor combination in a typical helmet visor display.
The conventional x-prism 24 is composed of four pieces of glass 26, 28, 30, 32 with a metal or dielectric coating on one side of each piece. These four pieces are bonded together to form an "X." Light entering the cube is split four ways, 25% to the right and the left, and 25% up and down. The light going up and down is wasted.
A conventional x-prism employs 50%/50% beamsplitter coatings, but since the rays from the CRT 20 must pass through these coatings twice before reaching the eye, the maximum theoretical efficiency is only 25% and typically the actual prisms are only 15% efficient. This low efficiency is due to the fact that if metal coatings are used for the conventional x-prism, there is considerable absorption; for dielectric coatings, the efficiency is tempered by the need for a wide angular bandwidth.
Another problem presented by helmet visor displays employing a conventional x-prism beamsplitter is glare. In normal day environment, light leaking up through the visor from one eye's side can pass through the x-prism and end up reflecting into the other eye, possibly washing out the scene. Here again, since light entering the cube from any face is split evenly into fourths, the 25% of the glare from one side (not counting other optical component losses) will end up at the other side of the visor display (see FIG. 2).
It is therefore an object of this invention to provide an x-prism with improved efficiency.
Another object is to provide an x-prism with improved glare characteristics.